There are several dive and snorkel sites around Ko Kut. However, the most interesting areas are around Koh Chang and Koh Rang Marine National Park south of Koh Chang (for divers and snorkelers). There is also the shipwreck HTMS Chang (for divers only) near Koh Chang, which is known for its abundance of fish. Whale sharks have also been spotted there!
Most places can be reached from Ko Kut, e.g. with BB Divers Ko Kut or with a speedboat as a day trip. Koh Rang is a popular excursion destination and you shouldn’t expect to be alone there.
On deepblu.com you will find a good overview of some dive spots in the area:
www.deepblu.com/planet/region/Gulf of Thailand – Ko Chang/TH/59f29eabd5487b52c5874783